This invention relates to metallic implants with load bearing surfaces coated with a thin, dense, low friction, highly wear-resistant, uniformly thick coating of oxidized zirconium.
The invention also relates to uniformly thick oxidized zirconium coatings on the non-load bearing surfaces of an orthopedic implant where the oxidized zirconium provides a barrier between the metallic prosthesis and body tissue thereby preventing the release of metal ions and corrosion of the implant. The invention further relates to a method of producing a uniformly thick oxidized zirconium layer by using an amorphous zirconium or zirconium alloy substrate with an altered surface roughness prior to formation of the oxide layer.
The excellent corrosion resistance of zirconium has been known for many years. Zirconium displays excellent corrosion resistance in many aqueous and non-aqueous media and for this reason has seen an increased use in the chemical process industry and in medical applications. A limitation to the wide application of zirconium in these areas is its relatively low resistance to abrasion and its tendency to gall. This relatively low resistance to abrasion and the tendency to gall is also demonstrated in zirconium alloys.
Orthopaedic implant materials must combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient of the implant is relatively young because it is desirable that the implant function for the complete lifetime of a patient. Of the conventional materials typically used to fabricate orthopaedic implants, each has its comparative advantages and disadvantages. In the case of metallic materials, because certain metal alloys have the required mechanical strength and biocompatibility without a high risk of brittle fracture, they are ideal candidates for the fabrication of prostheses. These alloys include 316L stainless steel, chrome-cobalt-molybdenum alloys and, more recently, titanium alloys which have proven to be the most suitable materials for the fabrication of load-bearing prostheses. However, metallic materials also have disadvantages. They are often not completely inert in the body. Body fluids act upon the metals causing them to slowly corrode by an ionizing process that thereby releases metal ions into the body. Metal ion release from the prosthesis is also related to the rate of wear of load bearing surfaces because the passive oxide film, which is formed on the surface, is constantly removed. The repassivation process constantly releases metal ions during the ionizing process. Furthermore, the presence of third-body wear (cement or bone debris) accelerates this process and microfretted metal particles increase friction against opposing surfaces. Surface hardness is not ideal in most metallic materials, resulting in scratching and microfretting. Other common materials have, other advantages and disadvantages. Ceramics, for examples, have very hard surfaces which resist scratching and microfretting, however, they are more brittle than metals and generally have inferior thermal properties.
In addition, the application of ceramic coating to metal substrates often results in non-uniform, poorly adhering coatings which tend to crack due to the differences in elastic modulus or thermal expansion between the ceramic and underlying metal substrate. Furthermore, such coatings tend to be relatively thick (50-300 microns) and since the bond between the metal and the ceramic coating is often weak, there is the risk of galling or separation of ceramic coatings.
The use of oxidized zirconium surfaces in orthopaedic implants represented an advance in that it allowed one to realize the advantages of metallic materials and ceramics while minimizing the disadvantages of both. Previous attempts have been made to produce oxidized zirconium coatings on zirconium parts for the purpose of increasing their abrasion resistance. One such process is disclosed in U.S. Pat. No. 3,615,885 to Watson which discloses a procedure for developing thick (up to 0.23 mm) oxide layers on various zirconium alloys. However, this procedure results in significant dimensional changes especially for parts having a thickness below about 5 mm, and the oxide film produced does not exhibit especially high abrasion resistance.
U.S. Pat. No. 2,987,352 to Watson discloses a method of producing a blue-black oxide coating on zirconium alloy parts for the purpose of increasing their abrasion resistance. Both U.S. Pat. No. 2,987,352 and U.S. Pat. No. 3,615,885 produce an oxidized zirconium coating on zirconium alloy by means of air oxidation. U.S. Pat. No. 3,615,885 continues the air oxidation long enough to produce a beige coating of greater thickness than the blue-black coating of U.S. Pat. No. 2,987,352. This beige coating does not have the wear resistance of the blue-black coating and is thus not applicable to many parts where there are two work faces in close proximity. The beige coating wears down more quickly than the blue-black oxide coating with the resulting formation of oxidized zirconium particles and the loss of the integrity of the oxidized zirconium surface. With the loss of the oxide surface the zirconium metal is then exposed to its environment and can lead to transport of zirconium joints away from the surface of the metal into the adjacent environment. U.S. Pat. Nos. 2,987,352 and 3,615,885 are incorporated by reference as though fully disclosed herein.
The blue-black coatings have a thickness which is less than that of the beige coating although the hardness of the blue-black or black coating is higher than that of the beige coating. This harder blue-black oxide coating lends itself better to surfaces such as prosthetic devices. Although the blue-black or black coating is more abrasion resistant than the beige coating it is a relatively thin coating. It is therefore desirable to produce the blue-black coatings of increased abrasion resistance without producing the same type coatings of the prior art.
U.S. Pat. No. 5,037,438 to Davidson discloses a method of producing zirconium alloy prostheses with a oxidized zirconium surface. This specific form of oxidized zirconium, described therein as blue-black or black oxidized zirconium is unique with respect to the excellent thermal conductivity it possesses relative to other conventional prosthetic materials. It combines excellent surface roughness characteristics with very high thermal conductivity. In this way, it possesses the relevant beneficial characteristics of metal and ceramics while avoiding the relevant disadvantages of the former and outperforming the latter. U.S. Pat. No. 5,037,438 is incorporated by reference as though fully set forth herein.
While the introduction of oxidized zirconium for medical implants represented an advance in this area, there was and is room for further refinements. While the durability of the oxidized zirconium implants is excellent compared to conventional materials, it has been recognized that high-integrity oxidized zirconium surfaces have even better durability. A high-integrity oxidized zirconium surface can be produced if the thickness of the oxidized zirconium layer has good uniformity. Non-uniformity of thickness negatively affects durability because it promotes the build-up of internal stresses in the oxide layer and such stresses tend to lead to cracks. Early on in the use of these surfaces for medical implants, uniformity of thickness was not controlled. It has been recognized that controlling uniformity of thickness results in a better oxidized zirconium surface for medical implant applications.
In U.S. Pat. No. 6,447,550, Hunter, et al. described a method for obtaining an oxidized zirconium coating of uniform thickness. Hunter taught that such is obtained by applying pre-oxidation treatment techniques to various zirconium-based materials that result in a refined microstructure and an altered surface roughness. Microstructure refinement is taught in the '550 patent by techniques which include the hot forge conversion of ingot to wrought barstock, closed die forging, rapid solidification, and powder consolidation. The altered surface roughness is accomplished by processes such as grinding, buffing, mass finishing, vibratory finishing, among others. U.S. Pat. No. 6,447,550 is incorporated by reference as though fully set forth herein.
Hunter, et al., in U.S. Pat. No. 6,585,772, provide another method for forming a uniformly thick oxide coating on zirconium or a zirconium alloy. In the method of the '772 patent, Hunter teaches that by inducing an altered surface roughness on a single phase/single composition zirconium based substrate prior to oxidation, control and improvement of the thickness uniformity of the resulting oxidized zirconium layer can be realized. The '772 patent also provides a method for forming a uniformly thick oxide coating on a zirconium or zirconium alloy prosthesis, for implantation in a patient, by inducing an altered surface roughness on at least a portion of the zirconium or zirconium alloy prosthesis, wherein the zirconium or oxidized zirconium consists, at least in part, of a single phase crystalline structure and uniform composition, prior to oxidizing the prosthesis to form a blue-black oxidized zirconium coating of uniform and controlled thickness on at least a portion of the surface of the prosthesis. U.S. Pat. No. 6,585,772 is incorporated by reference as though fully set forth herein.
While both of these techniques of Hunter proved useful in promoting thickness uniformity, the present invention provides yet another distinct method to accomplish the same result. In the present invention, a uniform thickness layer of oxidized zirconium is accomplished using an amorphous zirconium-containing alloy. This represents another technique to produce a surface layer of oxidized zirconium having improved uniformity of thickness and provides another tool in the arsenal of one wishing to fabricate improved medical devices having oxidized zirconium surfaces.